Glass sheets must be shaped to very exact dimensions when they are used as components of laminated aircraft windows or as pressing plates to press polish acrylic or polycarbonate sheets that are components of aircraft windows to insure that the major surfaces of the acrylic or polycarbonate sheets are smooth and free from wrinkles and other deviations from smoothness, such as extrusion marks that form during their fabrication. It is extremely important that the temperature within a kiln that is used to shape glass sheets be controlled with sufficient precision as to insure that one or more glass sheets stacked on a bending mold are bent to the curvature desired within precise limits of tolerance.
In the bending of such precision parts, it is important to have a uniform temperature within a kiln where a mold laden with one or more glass sheets is located for bending. Otherwise, the viscosity variations resulting from non-uniform heating would cause a deviation from desired shaping of the glass. Since glass sheets are supported in a horizontal plane for shaping by sag bending, it is important that the kiln temperature be substantially uniform from side to side and from end to end of the kiln, particularly at the level in the kiln at which one or more glass sheets are supported for sag bending.
It is also important that the temperature within a kiln be accurately controlled from outside the kiln so as to avoid the need to enter the kiln to change the atmospheric temperature. Some glass thicknesses and some glass shapes require different heating rates than others and it is also necessary to anneal shaped glass after it has been shaped. Thus, it is also important that the kiln be capable of fairly rapid temperature changes without causing a significant departure from substantially uniform temperature conditions throughout the extent of the kiln.
It is also important from the point of view of safety that the kiln in which glass sheets are bent be free of flames. Hot products of combustion that are completely burned in a combustion chamber outside the kiln are safer to handle within an enclosed kiln than open flames. Also, it is easier to obtain a more uniform horizontal temperature pattern with a gas delivery system that delivers a gas mixture containing combustion products than with a gas delivery system that delivers burning flames.